1. Field of the Invention
The present invention relates to an X-ray image tube, an X-ray image tube device and an X-ray device for displaying colors.
2. Description of the Related Art
Generally, in X-ray photography for use in medical diagnosis, industrial nondestructive inspection or the like, in order to improve sensitivity of a photographic system, an image is projected on the X-ray image tube, and the thus projected image is picked up by a monochrome CCD camera to pick up the X-rays. That is, in the X-ray photography, the X-rays passed through a subject is converted into visible light on an input fluorescent screen of an input side disposed in the X-ray image tube. This light is converted into electrons on a photoelectric surface disposed together with the input fluorescent screen. These electrons are then electrically amplified, and converted into visible light by a fluorescent substance of an output side to form an image. The thus-formed visible image is picked up by the CCD camera to form a visualized image of the subject.
In such X-ray photography, in order to improve visibility, a high-contrast image pickup state is widely utilized in which a gradient of a characteristic curve of the monochrome CCD is enlarged. For example, in the case of breast photography, it is necessary to photograph a calcified part, an abnormal soft tissue or the like of a small X-ray absorption difference with high resolution and high contrast.
However, under the above-mentioned photography conditions, in association with a dynamic range of measurement of the X-ray image tube being insufficient, there is a problem that even a slight deviation from the photography conditions makes it impossible to obtain an image of a proper density.
On the other hand, if objects of measurement are different in elemental composition, e.g., a bone and a muscle, it is necessary to set irradiation time of the X-rays by considering energy of the used X-rays, a thickness of the part, etc., and based on many experiences or the like. In this case, even if elemental compositions are substantially similar but densities are different, e.g., a normal tissue and an abnormal tissue such as a cancer, it is similarly necessary to set the irradiation time of the X-rays.
It is to be noted that color X-ray photographs changed in color corresponding to a difference in doses of an X-ray in color radiography are described in, for example, Jpn. Pat. Appln. KOKOKU Publication No. 48-6157 and Jpn. Pat. Appln. KOKOKU Publication. No. 48-12676. In the color X-ray photographs described in Jpn. Pat. Appln. KOKOKU Publication No. 48-6157 and Jpn. Pat. Appln. KOKOKU Publication No. 48-12676, a part of a small dose of the X-rays is colored by only a red component, a part of a lager dose of the X-rays is colored by adding a green component to the red component, and further a part of a large dose of the X-rays is colored by adding a blue component to the red component and the green component. Thus, coloring is carried out based on a difference in the doses of the X-rays.
However, even if information is extracted only from such a color change in the color X-ray photograph, for example, the green component and the blue component are added to the red component in the part of the large dose, and a color becomes similar to white in the color X-ray photograph. Consequently, accurate information extraction is difficult and, even if much information has been picked up, there is a problem that the information cannot be effectively used.
As a proposal considered effective for the above-mentioned problem, a color X-ray photograph capable of representing a plurality of colors corresponding to a dose of the X-rays is described in, for example, Jpn. Pat. Appln. KOKAI Publication No. 2001-209142. This publication discloses an emission sheet which emits a different color in accordance with a dose of a radiation such as the X-rays passed through a subject.
Additionally, in the above-mentioned publication, it is described that each of a plurality of colors emitted from the color emission sheet are detected by photodetecting means such as a color film or a color camera, and image information of a plurality of colors which have different sensitivity characteristics is obtained.
For the color emission sheet disclosed in the above-mentioned publication, there is used, for example, a fluorescent substance which comprises a main emission component corresponding to one luminescent color in a visible light range and at least one sub-emission component which has a luminescent color different from that of the main emission component and in which an emission ratio to the radiation of the same intensity is different from that of the main emission component. Moreover, in the above fluorescent substance to be used, an emission ratio between the main emission component and the sub-emission component is adjusted in accordance with a dynamic range of a photographic system.
Additionally, in the color X-ray photograph of the above-mentioned publication, relationships between an exposure and a concentration of the X-ray of the photographic system are set to be approximately equal intervals among colors (R, G and B) of a multicolor illuminant similar to that shown in FIG. 11. In an image obtained by using this illuminant, a part of a low dose is obtained as information of a red color of a state in which there is no information of a green color or a blue color. When the dose becomes large, the red color information is saturated to be obtained as green color information. When the dose becomes larger, the red color information and the green color information are saturated to become blue color information. That is, an image of a proper density is formed under relatively wide conditions for the dose of the X-ray, and a great deal of information can be obtained from the obtained image based on a difference on the doses of the X-ray. It is known that in emission characteristics of the multicolor illuminant shown in FIG. 11, inclination of a characteristic curve of the exposure to the concentration is gentle compared with emission characteristics of a general illuminant for monochrome image pickup.
However, in the color X-ray photograph disclosed in the above-mentioned publication, a dynamic range of output image is increased by expanding the dynamic range of the photographic system. Thus, there is a problem that proper image pickup conditions cannot always be obtained depending on a photography target part of a human body or an animal which is a subject, or an individual difference.
For example, there is a problem that even under proper conditions in breast photography of small X-ray absorption or the like, an image of a proper density cannot be obtained in trunk bone photography of large X-ray absorption or the like unless a dynamic range is matched. There is also a danger that the subject will be subjected to excessive X-ray exposure even if a dynamic range is in a suitable range.
There is a problem that in a photographic system range, even in a state in which each information of a red color (R), a green color (G) and a blue color (B) can be picked up as a proper image for a part of a small X-ray absorption difference such as that between a normal tissue and an abnormal tissue of a cancer or the like, for example, even if a proper image can be obtained by information of one of a red color, a green color and a blue color for a part of large X-ray absorption, e.g., a part of a large X-ray absorption difference for diagnosis from a bone to a blood vessel, the part of the small X-ray absorption difference cannot be obtained as a proper image.
Such a problem is not limited to the X-ray photography for medical diagnosis, but it similarly occurs in the X-ray photography for industrial nondestructive inspection. For example, between a case in which an object of measurement is iron and a case in which an object of measurement is plastic, optimal photography conditions caused by a difference in density are different, and a thickness or the like of a part to be photographed must be taken into consideration. Additionally, if a plurality of different materials such as composite materials are present, there is a problem that it is difficult to obtain a proper image.
On the other hand, a dynamic range of measurement in the color X-ray photograph is widened to enable acquisition of a great deal of information by one photography. However, when a combination of a scintillator responsive to the X-rays and a color CCD camera is compared with a combination of an X-ray image tube and a monochrome CCD camera, the sensitivity of the X-ray image tube which includes electronic amplification is ten times higher or more.
This means that in the color X-ray photograph obtained by the X-ray scintillator (illuminant layer) and the color CCD camera, a dose of the X-rays must be increased in order to obtain an image similar to that in the case of the combination of the X-ray image tube and the monochrome CCD camera. Thus, in the constitution described in Jpn. Pat. Appln. KOKAI Publication No. 2001-209142, there is a problem that the dose of the X-rays with which the subject must be irradiated is increased.
As described above, in the color X-ray photograph, the dynamic range of the photographic system is wide, proper photography conditions can be set in accordance with subjects of different X-ray transmission doses, occurrence of an exposure shortage, excessive exposure or the like caused even by slight deviation from the photography conditions or the like is reduced, and subject measurement can be assured more.
However, in order to make a density of an image proper, if a subject is clear, there is a problem that an emission characteristic curve of each color must be adjusted by changing an emission ratio of the color illuminant layer (scintillator layer) to match the subject. Additionally, in the color X-ray photograph, sensitivity lowers to one tenth or less compared with the monochrome X-ray image tube.
Further, in order to detect an image, it is necessary to use a lens of high transmittance or a camera of high sensitivity. It is to be noted that even if the lens of high transmittance or the camera of high sensitivity can be used, there is a problem that sufficient sensitivity cannot always be obtained.
Furthermore, for industrial use, there is a problem that it is difficult to identify a material of a large atomic number. Aside from this, for medical use, as the dose of the X-rays with which a subject should be irradiated, a dose of X-rays 10 which is larger by 10 to 1000 times compared with the monochrome X-ray image tube may be required. If the subject is a human body, there is a high possibility of nonusability.
In any case, in the color X-ray photograph, acquisition of static images has been realized, whereas acquisition of moving images has not been realized yet.